The invention relates to an apparatus and method for drying a transformer.
References known to the present applicant and believed to be possibly relevant to the present invention include the following U.S. Pats. Nos. 3,373,500 issued to Winter; 3,789,190 issued to Orosy, et al; 3,946,200 issued to Juodikis; and 3,959,692 issued to Wetzel. The Winter patent in particular teaches the necessity for drying transformers prior to filling with oil and the usefulness of internal heating of the transformers during evacuation to enhance the drying. The other patents in general teach various automatic temperature controls for resistive or heating elements. None of these patents actually provides a power transformer heating arrangement which allows the accurate heating of a transformer winding of unknown resistance to a preselected optimum temperature for drying.
As taught by the above referenced patents, it is important during the manufacture or repair of power transformers to be sure that the internal insulation is free of water before the unit is filled with oil and sealed. In the repair situation, a wide variety of transformers may be encountered having various winding resistances. While it is known that the winding resistance can indicate the internal temperature of the windings, it is necessary to know the cold resistance of a winding as well as to measure the resistance during heating since it is, in effect, the percentage increase resulting from the heating which indicates the temperature of the winding. Most available apparatus assumes that the windings are of a known cold resistance value or that the actual resistance will be measured and the appropriate high temperature resistance will then be calculated and set into the apparatus.
In the drying process the transformer casing is held at a very low pressure to enhance drying. The heating and evacuation are generally continued for a period of time sufficiently long to guarantee appropriate drying of transformer insulation for any type of transformer which may be encountered. It would be desirable to discontinue the heating and evacuation of the transformer as soon as an appropriate level of dryness has been reached to thereby reduce the total time required for drying.